RFC 6263

Internet Engineering Task Force (IETF) X. Marjou
Request for Comments: 6263 A. Sollaud
Category: Standards Track France Telecom Orange
ISSN: 2070-1721 June 2011 Application Mechanism for Keeping Alive the NAT Mappings
Associated with RTP / RTP Control Protocol (RTCP) Flows
Abstract
This document lists the different mechanisms that enable applications
using the Real-time Transport Protocol (RTP) and the RTP Control
Protocol (RTCP) to keep their RTP Network Address Translator (NAT)
mappings alive. It also makes a recommendation for a preferred
mechanism. This document is not applicable to Interactive
Connectivity Establishment (ICE) agents.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6263.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents
1. Introduction ....................................................22. Terminology .....................................................43. Requirements ....................................................44. List of Alternatives for Performing RTP Keepalive ...............44.1. Empty (0-Byte) Transport Packet ............................44.2. RTP Packet with Comfort Noise Payload ......................54.3. RTCP Packets Multiplexed with RTP Packets ..................54.4. STUN Indication Packet .....................................64.5. RTP Packet with Incorrect Version Number ...................64.6. RTP Packet with Unknown Payload Type .......................65. Recommended Solution for Keepalive Mechanism ....................76. Media Format Exceptions .........................................77. Timing and Transport Considerations .............................78. RTCP Flow Keepalive .............................................89. Security Considerations .........................................910. Acknowledgements ...............................................911. References ....................................................1011.1. Normative References .....................................1011.2. Informative References ...................................101. Introduction
[RFC4787] and [RFC5382] describe Network Address Translator (NAT)
behaviors and point out that two key aspects of NAT are mappings
(a.k.a. bindings) and keeping them refreshed. This introduces a
derived requirement for applications engaged in a multimedia session
involving NAT traversal: they need to generate a minimum of flow
activity in order to create NAT mappings and maintain them.
When applied to applications using the Real-time Transport Protocol
(RTP) [RFC3550], the RTP media stream packets themselves normally
fulfill this requirement. However, there exist some cases where RTP
does not generate the minimum required flow activity.
The examples are:
o In some RTP usages, such as the Session Initiation Protocol (SIP)
[RFC3261], agents can negotiate a unidirectional media stream by
using the Session Description Protocol (SDP) [RFC4566] "recvonly"
attribute on one agent and "sendonly" on the peer, as defined in
[RFC3264]. [RFC3264] directs implementations not to transmit
media on the receiving agent. If the agent receiving the media is
located on the private side of a NAT, it will never receive RTP
packets from the public peer if the NAT mapping has not been
created.

o Similarly, a bidirectional media stream can be "put on hold".
This is accomplished by using the SDP "sendonly" or "inactive"
attributes. Again, [RFC3264] directs implementations to cease
transmission of media in these cases. However, doing so may cause
NAT bindings to time out, and media won't be able to come off
hold.
o Some RTP payload formats, such as the payload format for text
conversation [RFC4103], may send packets so infrequently that the
interval exceeds the NAT binding timeouts.
To solve these problems, an agent therefore needs to periodically
send keepalive data within the outgoing RTP session of an RTP media
stream regardless of whether the media stream is currently inactive,
sendonly, recvonly, or sendrecv, and regardless of the presence or
value of the bandwidth attribute.
It is important to note that NAT traversal constraints also usually
require that the agents use Symmetric RTP / RTP Control Protocol
(RTCP) [RFC4961] in addition to RTP keepalive.
This document first states the requirements that must be supported to
perform RTP keepalives (Section 3). In a second step, the document
reports the different mechanisms to overcome this problem
(Section 4). Section 5 finally states the recommended solution for
RTP keepalive. Section 6 discusses some media format exceptions.
Section 7 adds details about timing and transport considerations.
Section 8 documents how to maintain NAT bindings for RTCP.
This document is not applicable to Interactive Connectivity
Establishment (ICE) [RFC5245] agents. Indeed, the ICE protocol,
together with Session Traversal Utilities for NAT (STUN) [RFC5389]
and Traversal Using Relays around NAT (TURN) [RFC5766], solves the
overall Network Address Translator (NAT) traversal mechanism of media
streams. In the context of RTP media streams, some agents may not
require all ICE functionalities and may only need a keepalive
mechanism. This document thus applies to such agents, and does not
apply to agents implementing ICE.
Note that if a given media uses a codec that already integrates a
keepalive mechanism, no additional keepalive mechanism is required at
the RTP level.
As mentioned in Section 3.5 of [RFC5405], "It is important to note
that keepalive messages are NOT RECOMMENDED for general use -- they
are unnecessary for many applications and can consume significant
amounts of system and network resources".

2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119
[RFC2119].
3. Requirements
This section outlines the key requirements that need to be satisfied
in order to provide RTP media keepalive.
REQ-1 Some data is sent periodically within the outgoing RTP session
for the whole duration of the RTP media stream.
REQ-2 Any type of transport (e.g., UDP, TCP) MUST be supported.
REQ-3 Any media type (e.g., audio, video, text) MUST be supported.
REQ-4 Any media format (e.g., G.711, H.263) MUST be supported.
REQ-5 Session signaling protocols SHOULD NOT be impacted.
REQ-6 Impacts on existing software SHOULD be minimized.
REQ-7 The remote peer SHOULD NOT be impacted.
REQ-8 The support for RTP keepalive SHOULD be described in the SDP.
REQ-9 The solution SHOULD cover the integration with RTCP.
4. List of Alternatives for Performing RTP Keepalive
This section lists, in no particular order, some alternatives that
can be used to perform a keepalive message within RTP media streams.
4.1. Empty (0-Byte) Transport Packet
The application sends an empty transport packet (e.g., UDP packet,
Datagram Congestion Control Protocol (DCCP) packet).
Con:
o This alternative is specific to each transport protocol.

4.2. RTP Packet with Comfort Noise Payload
The application sends an RTP packet with a comfort noise payload
[RFC3389].
Cons:
o This alternative is limited to audio formats only.
o Comfort noise needs to be supported by the remote peer.
o Comfort noise needs to be signaled in SDP offer/answer.
o The peer is likely to render comfort noise at the other side, so
the content of the payload (the noise level) needs to be carefully
chosen.
4.3. RTCP Packets Multiplexed with RTP Packets
The application sends RTCP packets in the RTP media path itself
(i.e., the same tuples for both RTP and RTCP packets) [RFC5761].
RTCP packets therefore keep the NAT mappings open as long as the
requirements for parameter selection are fulfilled as discussed in
Section 8.
Note: The "on hold" procedures of [RFC3264] do not impact RTCP
transmissions.
Cons:
o Multiplexing RTP and RTCP must be supported by the remote peer.
o Some RTCP monitoring tools expect that RTCP packets are not
multiplexed.
o RTCP must be configured so that the Tmin value [RFC3550] is less
than or equal to the Tr interval.

4.4. STUN Indication Packet
The application sends a STUN [RFC5389] Binding Indication packet as
specified in ICE [RFC5245].
Thanks to the RTP validity check, STUN packets will be ignored by the
RTP stack.
Con:
o The sending agent needs to support STUN.
4.5. RTP Packet with Incorrect Version Number
The application sends an RTP packet with a version number set to zero
(i.e., an incorrect version number).
Based on the RTP specification [RFC3550], the peer should perform a
header validity check and therefore ignore these types of packets.
Cons:
o Only four version numbers are possible. Using one of them for RTP
keepalive would be wasteful.
o [RFC4566] and [RFC3264] mandate that media with inactive and
recvonly attributes not be sent; however, this is mitigated, as no
real media is sent with this mechanism.
4.6. RTP Packet with Unknown Payload Type
The application sends an RTP packet of 0 length with a dynamic
payload type that has not been negotiated by the peers (e.g., not
negotiated within the SDP offer/answer, and thus not mapped to any
media format).
The sequence number is incremented by one for each packet, as it is
sent within the same RTP session as the actual media. The timestamp
contains the same value that a media packet would have at this time.
The marker bit is not significant for the keepalive packets and is
thus set to zero.
The synchronization source (SSRC) is the same as for the media for
which keepalive is sent.
Normally, the peer will ignore this packet, as RTP [RFC3550] states
that "a receiver MUST ignore packets with payload types that it does
not understand".

Cons:
o [RFC4566] and [RFC3264] mandate that media with inactive and
recvonly attributes not be sent; however, this is mitigated, as no
real media is sent with this mechanism.
o [RFC3550] does not preclude examination of received packets by the
peer in an attempt to determine if it is under attack.
o The statement "a receiver MUST ignore packets with payload types
that it does not understand" of [RFC3550] is not always observed
in real life.
o There is no RTCP reporting for the keepalive packets, as [RFC3550]
mandates that RTP packets with payload types that the receiver
does not understand be ignored.
o Some RTP payload formats do not handle gaps in RTP sequence number
well.
5. Recommended Solution for Keepalive Mechanism
The RECOMMENDED mechanism is that discussed in "RTCP Packets
Multiplexed with RTP Packets" (Section 4.3). This mechanism is
desirable because it reduces the number of ports when RTP and RTCP
are used. It also has the advantage of taking into account RTCP
aspects, which is not the case with other mechanisms.
Other mechanisms (Sections 4.1, 4.2, 4.4, 4.5, and 4.6) are NOT
RECOMMENDED.
6. Media Format Exceptions
When a given media format does not allow the keepalive solution
recommended in Section 5, an alternative mechanism SHOULD be defined
in the payload format specification for this media format.
7. Timing and Transport Considerations
An application supporting this specification MUST transmit either
keepalive packets or media packets at least once every Tr seconds
during the whole duration of the media session.
Tr has different value according to the transport protocol.
For UDP, the minimum RECOMMENDED Tr value is 15 seconds, and Tr
SHOULD be configurable to larger values.

For TCP, the recommended Tr value is 7200 seconds.
When using the "RTCP packets multiplexed with RTP packets" solution
(Section 4.3) for keepalive, Tr MUST comply with the RTCP timing
rules of [RFC3550].
Keepalive packets within a particular RTP session MUST use the tuple
(source IP address, source TCP/UDP port, target IP address, target
TCP/UDP port) of the regular RTP packets.
The agent SHOULD only send RTP keepalive when it does not send
regular RTP packets.
8. RTCP Flow Keepalive
RTCP packets are sent periodically and can thus normally keep the NAT
mappings open as long as they are sent frequently enough. There are
two conditions for that. First, RTCP needs to be used
bidirectionally and in a symmetric fashion, as described in
[RFC4961]. Secondly, RTCP needs to be sent frequently enough.
However, there are certain configurations that can break this latter
assumption.
There are two factors that need to be considered to ensure that RTCP
is sent frequently enough. First, the RTCP bandwidth needs to be
sufficiently large so that transmission will occur more frequently
than the longest acceptable packet transmission interval (Tr). The
worst-case RTCP interval (Twc) can be calculated using this formula
by inserting the max value of the following parameters:
o Maximum RTCP packet size (avg_rtcp_size_max)
o Maximum number of participants (members_max)
o RTCP receiver bandwidth (rtcp_bw)
The RTCP bandwidth value to use here is for a worst case, which will
be the receiver proportion when all members except one are not
senders. This can be approximated to be all members. Thus, for
sessions where RR and RS values [RFC3556] are used, then rtcp_bw
shall be set to RR. For sessions where the [RFC3550]-defined
proportions of RTCP bandwidth are used (i.e., 1/4 of the bandwidth
for senders and 3/4 of the bandwidth for receivers), then rtcp_bw
will be 5% of 3/4 of the AS value [RFC4566] in bits per second.
Twc = 1.5 / 1.21828 * members_max * rtcp_bw / avg_rtcp_size_max * 8

The second factor is the minimum RTCP interval Tmin defined in
[RFC3550]. Its base value is 5 seconds, but it might also be scaled
to 360 divided by the session bandwidth in kbps. The Extended RTP
Profile for Real-time Transport Control Protocol (RTCP)-Based
Feedback (RTP/AVPF) [RFC4585] also allows for the setting of a
trr-int parameter, which is a minimal RTCP interval for regular RTCP
packets. It is also used as the Tmin value in the regular Td
calculation. An analysis of the algorithm shows that the longest
possible regular RTCP interval is:
RTCP_int_max = trr-int * 1.5 + Td * 1.5 / 1.21828
And as long as there is sufficient bandwidth according to criteria 1
below, then the algorithm can be simplified by setting Td = trr-int,
giving
RTCP_int_max = trr-int * (1.5 + 1.5 / 1.21828) = 2.73123 * trr-int
Thus, the requirements for the RTCP parameters are as follows for
functioning keepalive:
1. Ensure that sufficient RTCP bandwidth is provided by calculating
Twc, and ensure that the resulting value is less than or equal
to Tr.
2. If AVP or SAVP [RFC3711] is used, the Tmin value can't be greater
than Tr divided by 1.5 / (e-3/2).
3. If AVPF or SAVPF [RFC5124] is to be used, trr-min must not be set
to a value greater than Tr / 3.
9. Security Considerations
The RTP keepalive packets are sent on the same path as regular RTP
media packets and may be perceived as an attack by a peer. However,
[RFC3550] mandates that a peer "ignore packets with payload types
that it does not understand". A peer that does not understand the
keepalive message will thus appropriately drop the received packets.
10. Acknowledgements
Jonathan Rosenberg provided the major inputs for this document via
the ICE specification. Magnus Westerlund provided the text for the
RTCP flow keepalive section. In addition, thanks to Alfred E.
Heggestad, Colin Perkins, Dan Wing, Gunnar Hellstrom, Hadriel Kaplan,
Randell Jesup, Remi Denis-Courmont, Robert Sparks, and Steve Casner
for their useful inputs and comments.